Linda R. Chamberlain

Reactivity of tantalum η2-iminoacyl groups: intramolecular coupling, reduction, and dealkylation

The reaction of arylisocyanides with a number of mixed alkyl, aryloxides of tantalum allows the isolation of the corresponding iminoacyl derivatives which can further react to give a range of products; the structures of the complexes obtained have been confirmed by X-ray diffraction.

Synthesis and solid state structure of tetra(2,6-di-isopropylphenoxy)-hexamethoxy-di-tantalum(V), Ta2(OC6H3Pri2)4(OCH3)6

Abstract The reaction of Ta(OAr-2,6-Pri2)2(CH3)3(OAr-2,6- pri2=2,6-di-isopropylphenoxide) with oxygen in hexane solvent has allowed the isolation of the mixed alkoxide, aryloxide Ta2(OAr-2,6-Pri2)4(OCH3)6 (1) as large white crystals which have been studied by X-ray diffraction methods. Crystal data for Ta2C54- H86O10, Mr=1257.16, monoclinic, P21/a, a= 22.137(7), b=10.494(3), c=12.452(4) A, β= 105.09(2)°, V=2792(2) ,A3, Z=2, Dx=1.495 g cm−3, Mα, λK=0.71069 A,μ=39.2 cm−1, F(000) =636, T=112 K,R=0.030 for 3212 unique observed reflections. The structure consists of an octahedral arrangement of oxygen donor atoms around two tantalum metal centers in an edge-shared bi-octahedron with two bridging methoxide ligands. The other four methoxide and four phenoxide ligands are terminal.

The synthesis and structure of Group 5 metal alkyl and alkylidene complexes containing 2,6-dialkylphenoxide ligands: X-ray crystal structures of [Ta(OC6H3Me2-2,6)2(CH2Ph)3], [Ta(OC6H3Me2-2,6)4Me], and [Ta(OC6H3But2-2,6)2(CHSiMe3)(CH2SiMe3)]

A number of alkyl and alkylidene complexes of niobium and primarily tantalum containing 2,6-dialkylphenoxide ligation, OC6H3R2-2,6 (R = Me, Pri, or But) have been synthesized and characterized. For the sterically less demanding 2,6-dimethyl- and 2,6-di-isopropyl-phenoxides a series of compounds [Ta(OC6H3R2-2,6)2(CH2R′)3](R′= H, SiMe3, or Ph) can be readily obtained by alkylation of the corresponding mixed chloro-aryloxide. The remaining two aryloxides are more resistant to further substitution. Spectroscopic evidence supports the σ-bonded compounds adopting a trigonal-bipyramidal structure with trans-axial aryloxide ligands. This structure was confirmed by a single-crystal X-ray diffraction study on [Ta(OC6H3Me2-2,6)2(CH2Ph)3]. The di- and mono-alkyls [Ta( OC6H3Me2-2,6)3( CH2Ph)2] and [Ta(OC6H3Me2-2,6)4Me] have also been isolated and the latter shown to adopt a square-pyramidal structure in the solid state with an axial methyl group. In contrast the reaction of [Ta(OC6H3But2-2,6)2Cl3] with Li(CH2SiMe3) or Mg(CH2Ph)2 leads to the mixed alkyl–alkylidene complexes [Ta(OC6H3But2-2,6)2(CHR′)(CH2R′)](R′= SiMe3 or Ph). The presence of the alkylidene functional group was confirmed both spectroscopically and by a single-crystal X-ray diffraction study on the trimethylsilyl derivative. A related complex, [Ta(OC6H3Pri2-2,6)2(CHSiMe3)(CH2SiMe3)], was obtained by photolysis of [Ta(OC6H3Pri2-2,6)2(CH2SiMe3)3].

Synthesis and intramolecular coupling of tantalum η2-iminoacyl (η2-RNCR') and η2-imine (η2-RNCR'2) functional groups

Abstract The migratory insertion of organic isocyanides into the metal-carbon bonds of a series of tri-alkyls, Ta(OAr)2R3 (OAr = 2,6-dimethylphenoxide; R = CH3, CH2Ph), has been examined. Addition of 2,6-dimethylphenyl- or 2,6-di-isopropylphenylisocyanide (Ar′NC ⩾ 2 equivalents) results in the essentially quantitative formation of the bis-insertion products, Ta(OAr)2(η2-Ar′NCR)2R (1). Spectroscopic as well as structural data are consistent with both iminoacyls being η2-bound to the metal centre. In contrast, addition of tert-butylisocyanide to the tris-aryl Ta(OAr)2Ph3 proceeds to produce a mixed η2-imine, η2-iminoacyl compound Ta(OAr)2(η2 -ButNCPh2)(η2ButNCPh) (2). This compound is presumably formed by a second phenyl group migration taking place in an intermediate of type (1). All of these compounds exhibit thermal instability to varying degrees. The methyl compound Ta(OAr)2(η2-Ar′NCMe)2(Me) rearranges over days at 25°C to produce a mixed amido, imido complex Ta(OAr)2(Ar′NCMeCMe2)(NAr′) (3a). This is believed to be formed by intramolecular coupling and fragmentation in an intermediate, Ta(OAr)2(η2-Ar′NCMe2)(η2-Ar′NCMe) of type (2). Support for this hypothesis comes from thermolysis of (2) to produce Ta(OAr)2(ButNCPhCPh2)(NBut) (3b) which has been structurally characterized. The benzyl derivatives Ta(OAr)2(η2 -Ar′NCCH2Ph)2(CH2Ph) also thermally produce mixed imido, amido compounds, only this time more dramatic fragmentation occurs to produce Ta(OAr)2(Ar′NCH2CH2Ph)(NAr′) (4) and an organic fragment, PhCH2CCPh. Structural studies confirmed this formulation. Mechanistic aspects of the reactivity of these η2-iminoacyl and η2-imine functionalities are discussed. Crystal data for TaC48H61N2O2 (3b) at −158°C are a = 21.806(5), b = 19.568(6), c = 10.078(2)A, β = 90.63(1)°, Z = 4, dcalc, = 1.358 g cm−3, in space group P21/n. Of the 5642 unique intensities collected with Mo-Kα, the 4790 with F> 2.33σ(F) were used in the least-squares refinement to yield R = 0.0419, Rw = 0.0439. Crystal data for TaC44H51N2O2 (4) at −155°C are a = 12.423(5), b = 20.772(10), c = 14.840(6) A, β = 93.27(2)°, Z = 4, dcacl = 1.426 g cm−3 in space group P21/n. Of the 5004 unique intensifies collected with Mo-Kα, the 3209 with Fo> 3.00 σ(F) were used in the least-squares refinement to yield R = 0.0574, Rw = 0.0498.

Electronic absorption spectra and photochemical reactivity of Group 5 metal alkyl compounds: photochemical α-hydrogen abstraction

The electronic absorption spectra as well as the electrochemical and photochemical behaviour of a number of niobium and tantalum alkyls of formula [MX2(CH2R′)3](X = Cl, OPri, OC6H3Me2-2,6, OC6H3Pri2-2,6, and OC6H3But2-2,6; R′= H, SiMe3, or Ph) have been studied. The electronic spectra of these d0 compounds are dominated by an intense ligand to metal charge transfer (l.m.c.t.) band, the energy of which is strongly dependent on X. Evidence is presented in support of the assignment of these bands as alkyl to metal in character. Furthermore, the observed photochemistry of these compounds, in some cases leading to the efficient and almost quantitative photosynthesis of alkylidene functional groups, has been investigated. For [Ta(OC6H3But2-2,6)2Me3] irradiation into the observed l.m.c.t. band at 313 nm produces the methylidene complex [Ta(OC6H3But2-2,6)2-(CH2)(CH3)] and methane with a quantum efficiency of 0.95 ± 0.1. Mechanistically, studies indicate that the reaction is concerted while for [Ta(OC6H3Pri2-2,6)2(CH2SiMe3)3] photogeneration of the corresponding alkylidene involves an intermediate alkyl radical which can be intercepted.

Crystal and molecular structure of monochlorotris (2,6-di-t-butylphenoxo)hafnium

The compounds MCl(OC6H3But2-2,6)3(M = Zr, Hf) have been synthesized and the hafnium compound has been structurall characterized to show a sterically congested, mononuclear molecule in which several distortion of the aryloxo-ligands have taken place.